The present invention relates generally to three-dimensional (3D) computerized tomography (CT) and, more particularly, the present invention relates to a scanning method and system for imaging relatively large objects with relatively small area detectors.
In conventional computerized tomography for both medical and industrial applications, an x-ray fan beam and a linear array detector are used. Two-dimensional (2D) imaging is achieved. While an acquired data set may be complete and image quality is correspondingly high, only a single slice of an object is imaged at a time. When a 3D image is required, a stack of slices approach is employed. Acquiring a 3D data set one 2D slice at a time is inherently slow. Moreover, in medical applications, motion artifacts occur because adjacent slices are not imaged simultaneously. Also, dose utilization is less than optimal because the distance between slices is typically less than the x-ray collimator aperture, resulting in double exposure to many parts of the body. In 2D CT, the scanning path of the source is often a simple circular scan about the object. The linear array detector is fixed relative to the source. (Although it is usual to talk about a scan path of a source relative to the object to be imaged, it is to be appreciated that the object may be rotated or otherwise moved to provide relative motion between the object and the source.)
In a system employing true cone beam geometry for 3D imaging, a cone beam x-ray source and a 2D area detector are used. An object is scanned, preferably over a 360.degree. angular range either by moving the x-ray source in a scanning circle or other path about the object or by rotating the object while the source remains stationary. In either case, the area detector is fixed relative to the source. The relative movement between the source and object which is to be imaged provides scanning in either case. Compared to the conventional 2D stack of slices approach to achieve 3D imaging, the cone beam geometry has the potential to achieve rapid 3D imaging of both medical and industrial objects with improved dose utilization.
The 2D area detector used for 3D imaging generally has detector elements arranged in rows and columns. Such area detectors, using an array of detector elements, have had either flat or curved geometry. In other words, the rows and columns have been arranged in a plane for flat geometry detectors and have been arranged in curves for curved geometry detectors. Available area detectors have generally been of large size and low quality, such as x-ray image intensifiers, or of small size and higher quality. High costs and other factors have made high quality, high resolution, large area 2D array detectors generally unavailable.
U.S. Pat. No. 5,032, 990, issued Jul. 16, 1991, entitled "TRANSLATE ROTATE SCANNING METHOD FOR X-RAY IMAGING," assigned on its face to the assignee of the present application, and hereby incorporated by reference, discloses a technique for two-dimensional imaging of an object which is so wide that a linear array detector is not wide enough to span the object or part which is to be viewed.
U.S. Pat. No. 5,187,659, issued Feb. 16, 1993, entitled "CONE BEAM SCANNING TRAJECTORIES FOR THREE-DIMENSIONAL COMPUTERIZED TOMOGRAPHY DATA ACQUISITION WHERE OBJECT IS LARGER THAN THE FIELD OF VIEW," assigned to the assignee of the present application, and hereby incorporated by reference discloses a technique for avoiding corrupted data when performing 3D CT on an object larger than the field of view.
U.S. patent application Ser. No. 07/998,330, filed Dec. 30, 1992, U.S. Pat. No. 5,319,693, in the name of Eberhard et al., entitled "THREE-DIMENSIONAL COMPUTERIZED TOMOGRAPHY SCANNING CONFIGURATION FOR IMAGING LARGE OBJECTS WITH SMALLER AREA DETECTORS", assigned to the assignee of the present application, and hereby incorporated by reference, discloses a technique for three-dimensionally imaging relatively large objects using relatively small area detectors by changing the configurations corresponding to the relative positioning of a source of cone beam energy, the object which is to be imaged, and the area detector.
Kudo and Saito (hereinafter Kudo) disclose a helical scanning technique in an article entitled "Three-Dimensional Helical Scan Computed Tomography Using Cone-beam Projections", Journal of Systems and Computers in Japan, Vol. 23, No. 12, pp. 75-82 (1992) and hereby incorporated by reference. However, Kudo fails to disclose in the above article how such technique can be effectively used for exactly reconstructing the image of the object under inspection being that Kudo's technique at best only provides an approximate reconstruction technique. Further, although Kudo discloses a helical scanning technique which can be utilized to reduce a predetermined dimension (such as height or width) of the area detector such reduction is limited to at least twice the helix pitch spacing between consecutive stages formed by a given helical scan path assuming a constant helix pitch spacing. Thus, Kudos's scanning technique does not suggest how to reduce such predetermined dimension by two to one so as to span just the spacing between any two consecutive stages of the helical scanning path, again assuming a constant helix pitch spacing. (No representation is made or intended that this referenced article or previously referenced application or issued patents are necessarily prior art to the present application.)